Morphometric and autoradiographic methods will be used to evaluate proliferative activity and morphogenesis in pulmonary mesenchyme during late fetal development. Population doubling time will be used as a measure of growth and volume density of air space and of capillaries will be used to evaluate morphogenesis. These parameters will be compared in normal lungs and in lungs of fetuses exposed transplacentally to dexamethasone. The lungs of fetuses exposed transplacentally to dexamethasone. The lungs of mice similarly exposed to dexamithasone during prenatal development will also be examined during the early post-natal growth period in order to evaluate possible alterations in mesenchymal proliferation and morphogenesis, such as the outgrowth of secondary (alveolar) septae.